The unique feature of our research is to develop the methods necessary for long term clinical studies of environmental heat transfer, water balance and nutrient utilization in small premature infants. We aim to test the hypothesis that their growth may be compromosed in adverse thermal environments; as a corollary, it may be possible to improve growth rate by appropriate intervention in the thermal environment. Our specific objectives are: 1. To develop a whole body canopy to measure total metabolic energy production, thermal energy exchange, water loss, and respiratory gas exchange between the infant and his microenvironment. 2. To develop methods which utilize nitrogen balance, the [15N] enrichment of plasma lysine and the urinary excretion of 3-methylhistidine to characterize growth, total body protein turnover and skeletal muscle degradation. 3. To combine the whole body canopy and nitrogen metabolism procedures so as to determine the effects of simple changes in the thermal environment upon resting metabolism and growth in the clinical setting. In particular, we will compare the use of a convection heater to a radiant heater and the unclothed to the clothed infant. Advances in perinatal care over the last decade have significantly decrased the mortality of low-brith-weight infants. For those who survive the immediate postnatal period, attention must now be directed to ensuring normal developmetal outcome. We suggest that a better understanding of energy requirements and energy expenditure will improve our ability to provide optimal nutrition for these infants. In particular, improved control of the infant's environmental temperature should lead to a noticeable decrease in caloric drain and a net increase in positive energy balance. It is hoped that this initial study will allow the developoment of specialized computational and instrumental methods that can later be extended to the study of the distressed infant and the more subtle effects of drugs, intravenous alimentation and infant activity.